fix(composer): handling of plain and html bodies

[st3iny]

Fix #10747

Things I tested

1. Compose a new message and send it.
2. Edit a message in the outbox and send it again.
3. Toggle the editor mode (plain vs. rich text).
4. Reply, forward, edit as new.

[SebastianKrupinski]

Question, since you are reworking this, now that we have the htmlBody and plainBody, can we get ride of the editorBody?

[st3iny]

Question, since you are reworking this, now that we have the htmlBody and plainBody, can we get ride of the editorBody?

Yes, we should be able to get rid. However, I'd move this to another PR.

[ChristophWurst]

/backport to stable4.3

[ChristophWurst]

/backport to stable5.0

[st3iny]

One thing to note: I didn't check if automated message are not editable. Sebastian told me that they can only be detected by their missing editorBody field. However, my logic always relies on htmlBody and plainBody.

[SebastianKrupinski]

Tested. All function in description seem to work correctly

[ChristophWurst]

4.2 backport stopped